Venous stent

ABSTRACT

This invention is a modular intraluminal tubular stent system for deployment in the venous system and a method of stenting a vein segment using a modular stent system. One of the stents in the system includes a reinforced terminal end portion, such as a helical coil, to provide additional expansive force to maintain the initial deployed location of the stent. The coil may be interwoven into the wall of the stent, or a separate structure deployed separately.

PRIORITY STATEMENT

This application is a continuation of U.S. application Ser. No.13/198,917 filed Aug. 5, 2011, which was a continuation of U.S.application Ser. No. 12/903,056 filed Oct. 12, 2010, which was acontinuation of U.S. application Ser. No. 12/603,970 filed Oct. 22,2009, which was a continuation in part of U.S. application Ser. No.11/944,094 filed Nov. 21, 2007, which claimed the priority benefit ofU.S. provisional application No. 60/866,742 filed Nov. 21, 2006.Applicant hereby claims priority to all these applications, and thecontents of which are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to endoluminal tubular stents,such as stents and other structures. More particularly, the presentinvention provides modular tubular stent structures having propertieswhich can be tailored for individual body lumens, including bloodvessels, and more particularly, for placement in the venous system.

Stents and related endoluminal devices are currently used by medicalpractitioners to treat portions of the aorta and peripheral arterialvascular system that have become weakened, developing an aneurysm (aweakening of the artery wall resulting in a distended arterial sectionthat is subject to rupture), or that have become, or portions of thevenous vascular system that have become so narrowed that blood flow isrestricted (commonly referred to as “stenosis”).

Stents are generally cylindrically shaped devices which function toexpand when deployed. Stents may be balloon expandable or selfexpanding. The balloon expandable stent is a stent that is usually madeof a coil, mesh or zigzag design. The stent is pre-mounted on a balloonand the inflation of the balloon plastically expands the stent withrespect to the balloon diameter. Self-expanding stents are a tubulardevice stored in an elongate configuration in what is called a deliverysystem or applicator. The applicator is introduced percutaneously intothe body into a vessel at a suitable location, and guided through thevessel lumen to the location where the stent is to be released. Thedelivery system and the stent are often provided with radiologicalmarkers with which the positioning and the release of the stent can bemonitored in situ under fluoroscopy. Upon release, the stent materialauto expands to a predetermined size.

Commonly used self-expanding stents are braided stents, or laser cutstents. A braided stent is a metal stent that is produced by what iscalled a plain weaving technique. It is composed of a hollow body, whichcan stretch in the longitudinal direction and whose jacket is a braidmade up of a multiplicity of filament-like elements which, in theexpanded state of the braided stent, intersect a plane, perpendicular tothe longitudinal direction, at a braid angle. A braided stent undergoesa considerable change in length when deployed (“foreshortening”), thischange in length being all the greater the greater the original diameterand the smaller the original braid angle (e.g. Wallstent from BostonScientific (Boston, Mass. USA)). Because of the considerable shorteningthat takes place upon release of a braided stent, precise placement isdifficult. Laser cut stents are constructed from a tube of material(most frequently, nitinol (a nickel titanium alloy), and also stainlesssteel, cobalt, etc) that is laser-cut during production to create ameshed device. The tube is comprised of sequential aligned annular ringsthat are interconnected in a helical fashion. The tube is compressed andloaded into the delivery device and expands to original size whenreleased. Nitinol, which has thermal memory, may help stents made ofthis material expand into position when exposed to body temperatureafter delivery. Compared with self-expanding braided stents, laser cutstents provide more accurate stent deployment with less foreshortening.Laser cut stents are much less subject to foreshortening and areprobably less rigid than braided stents.

For treatment of aneurysms, the stent generally includes a graft orliner, or may be a liner alone with a stent-like device at each end ofthe stent for sealing against healthy aortic vessel walls away from theweaken aortic section. The graft or liner is generally made of inelasticnon-expanding material, and is generally impermeable to blood, as thestent-graft is intended to prevent blood flow through the liner and intothe surrounding artery. Graft material is generally a non-selfsupporting fabric material that must be attached to the stent frame forsupport. The combination of a liner and a stent limits the possibleradial expansion of the stent, as the liner material is generallyinelastic. Constrained expansion of a stent by the liner is desired inthe aortic system, as it is not desired to have the liner-stent sealagainst unhealthy distended aortic tissue—rather, the liner stent isdeployed to create a sealed passage through the weakened aortic sectionand to prevent blood flow between the exterior surface of theliner/stent combination (or liner alone) and the weaken aortic walls,thereby preventing the possible rupture or bursting of the weakenedaortic walls.

In the venous system, the setting is different—the issue is not weakenedvessel walls, but stenosis within the vessel. Venous stenosis may becaused by clotting, scarring following blood clots or by focal externalcompressive forces on a venous vessel (such as in the femoral vein whereit crosses the inguinal ligament or in the pelvic vein where it iscrossed by overlaying pelvic arteries). In treating a venous stenosis, aliner or graft is not necessary; indeed, a liner or graft is notpreferred, as the stent must function to expand against the narrowedvein section, thereby expanding the narrowed section to a more normalcross-sectional area. It is not desired to limit expansion a stent usedfor treating stenosis in the venous system.

While stents in the venous system are most often used to “prop open”blood vessels, they can also be used to reinforce collapsed or narrowedtubular structures in the body, such as the respiratory system, thereproductive system, or any other tubular body structure. These stentsare generally mesh-like so that endothelial and other cells can growthrough the openings integrating the stent into the venous wall andsometimes resulting in restenosis of the tubular structure. Inclusion ofliners or grafts would prevent the integration of the stent into thevenous wall. Typically, one or both ends of the stent is flared in orderto facilitate anchoring within the vessel.

Most stents are designed to work in fairly small lumens and arerelatively short in length. However, lumens in the venous system can bemuch larger than aorta and peripheral arteries and the desired stentlength can be long in comparison to arterial stents. Both features ofthe venous system present problems for conventional stent design, wherethe conventional stent structure is typically formed with cylindricalframes having axially constant diameters and constant expansive forcesalong their lengths. Additionally, long length stent structures may alsoencounter variations in lumen size over the venous application length,making placement and use of a single sized cross-sectional sized stentproblematic. One method to accommodate the different lumen diameters inthe aortic system is with a modular stent system as shown in U.S. Pat.No. 6,193,745, hereby incorporated by reference. However, in this stentsystem, compressive/expansive forces on one modular section tend toshorten or lengthen the particular section, allowing for relativemovement between adjacent modular stent sections, not desired in avenous system application. Such movement is not desirable, particularlywhere proper stent placement is critical to accommodate intersectingveins. Additionally, this system is designed for aneurysms, and hence,liners are employed.

In order to overcome some or all of these drawbacks, a stent system isneeded that can account for the difficulties of placement within thevenous system and to accommodate variations in geometry along bodylumens without compromising the effectiveness of the stent. It wouldfurther be desirable to provide adaptable modular stents and methods fortheir placement which would facilitate effective treatment of widelyvarying luminal system geometries without requiring the maintenance of alarge inventory of stent module models.

2. Summary of the Invention

The present invention provides modular intraluminal tubular stents fordeployment in the venous system. The stents can be utilized in a modularsystem, allowing placement of multiple overlapping stents to form acomposite stent structure having characteristics which are tailored tothe specific requirements of the patient. A particular modular stent mayhave an opening or fenestra in the sidewall to accommodate flow from aside vein that joins the vein where the modular stent is positioned. Amodular stent may have reinforced portions where the stent side wallmaterial is varied for particular reasons, for instance, to addreinforcement to a portion of the stent that is subject to greater focalcompressive/expansive radial body forces (a compressive force is oneapplied that tends to bend the stent or collapse the stent inwardly).Additionally, a stent may have a reinforced terminal end to provideadditional expansive force to maintain the initial deployed location ofthe stent. The reinforced section extends from near one end of the stentto suitable distance back from that end, for instance form several mm to40 mm, but generally not more than ¼ or at most, ½ the length of thestent. Modification of the construction of stent sidewall materials(such as by the addition of additional reinforcing expansive rings orvariable geometry) can be made in desired portions of the stent toprovide a “customized” modular stent section that can have varying axial(lengthwise) properties, or a helical expansive ring may be joined withthe stent (e.g. attached to the stents interior or exterior sidewall, orinterwoven into the side wall). A separate reinforcement stent (a smallstent having either greater expansion characteristics or greaterresistance to compressive forces) or a separate helical coil may also beused in conjunction with a venous stent or venous stent module. Stentmodules may be combined to form longer stent structures as needed to fitthe needs of individual patients.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction showing a four stent module application (module A,B, C, and IVC Module) deployed in the pelvic venous system.

FIG. 2A is a depiction of module A depicted in FIG. 1, showing a sideaperture and tapered body, and non-reinforced lower end.

FIG. 2B is a depiction showing module A being deployed from the distalend of a catheter.

FIG. 3A is a depiction showing module B with both ends non-reinforced orunsealed.

FIG. 3B is a depiction showing module B being deployed from the distalend of a catheter.

FIG. 4A is a depiction showing stent module C having tapered body, andreinforce section, sealed lower end and unsealed upper end.

FIG. 4B is a depiction showing deployment of module C from the proximalend of a catheter.

FIG. 5 is a depiction of the IVC stent module.

FIG. 6A is a schematic showing the relationship of an interior stent,exterior stent and reinforcing coil.

FIG. 6B is a cross section through the assembled stents of FIG. 6A,showing the geometry of the completed stent system.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Construction details for stents are well known in the art. The stentmodules will generally be radially expandable from a narrow-diameterconfiguration to facilitate introduction into the body lumen viacatheter, and are constructed without a liner or graft. The tubular orcylindrical structure generally comprises a series of independent ringsthat are interwoven or interlaced to create an elongated tubularstructure. Alternatively, the stent can be constructed as a braidedtubular structure, much like a Chinese finger. By varying the braidpattern or density of the braids (or braid material) or the rings, orthe braid or ring material, a particular stent module can be customizedas desired for varying the expansive force, or for varying theresistance to compressive forces, along the length of the stent.

The operational requirements of a specific venous stent structure mustaccount for the environmental placement. Venous stents are most likelyto be employed in the pelvic veins, and commonly extend from theinferior vena cava (typical size about 21 to 25 mm average) to a landingsite below. The caudal landing site may be in the common iliac vein(14-16 mm size average), external iliac vein (12-14 mm size average) orthe common femoral vein below the inguinal ligament. To accommodatediffering caudal landing sites, and therefore the different lengths, thedesign should be modular to account for differing lumen diameters. Oneparticular embodiment is shown in FIG. 1. Shown is a four stent moduleapplication. The stent structure starts at the inferior vena cava (IVC)which is the largest of the veins for this particular application, andends in a much smaller vein below. The first stent module extends fromthe vena cava to the common iliac vein (module A), the second stentmodule extends from the common iliac vein to the external vein band(module B) the third stent module extends from the external iliac veinto the common femoral vein (module C) (these stent modules can be usedalone or in combination as needed). Typical stent diameters and lengthsfor the venous system are indicated in the drawings.

When two or more stent modules are employed together to create a longstent structure, an overlap is desired between the adjacent stents(generally an overlap 3 to 4 cm can be utilized). Hence, the length ofeach module does not have to be exact and conform to the anatomicalconditions, but can be somewhat shorter or somewhat longer, even stentswith deployed diameters larger than then native vein segment may beused. The differences in adjacent overlapped stent segments will beaccounted for in the overlapped region. The uppermost module, ifstarting in the vena cava, needs to accommodate flow that enters fromthe opposite iliac vein. An aperture, opening or fenestra is thusconstructed into the sidewall of the tubular stent body for thispurpose. As bilateral stents are sometimes necessary, the fenestra canalso be employed for placement of an intersecting or joining stent.

There are several pressure points along the course of these veins, andenforcement of the stents at these points are desirable. Reinforcementcan be accomplished by placing a reinforcing liner stent (a separatestent, constructed without a liner or graft, and used to line a portionof a modular stent section) on the interior of a pre-positioned deployedstent module or a reinforcing coil stent, such as a helical coiledspring. Alternatively, the coil structure may be integrated into thestent wall, or, a reinforcing stent module (e.g., a reinforcement stent)can be first positioned in the vein and a second stent module positionedthrough and interior to the reinforcement stent (and generally extendingbeyond) the reinforcement stent. Finally, the stent module may have areinforcing section constructed within or interwoven with, the tubularstructure, such as by varying the construction materials of the stent inthe portion to be reinforced (e.g., adding additional and closer spacedrings to the stent tubular body, varying the construction materials(stiffer materials), etc.). Stent body module construction may furthervary a number of stent characteristics, including length, cross-section,flexibility, resilient spring force, and conformability.

When the stent is compressed by pressure points in the veins, the stentbody will tend to shorten or lengthen, and the stent may contract at theupper and lower ends of the stent. With stents constructed of interwovenspiral metal wires, such movement is more likely. This movement isundesirable as placement of the stent ends can alter the criticalplacement of the stent components (such as the movement of a sidefenestra's placement). In a modular stent system, this movement canalter the degree of overlap of adjacent stents, and if overlap is notsufficient, could result in an unstented section within the extendedstent system, an undesired condition, as such unstented sections aremore prevalent to developing recurrent blockage. In overlapped stentmodules, one module is exterior (exterior surface in the overlappedregion facing the vessel sidewalls), while the other stent module is theinterior stent (exterior surface in then overlapped region faces theinterior surface of the exterior stent. To prevent movement of adjacentstents, overlapped sections can be coupled or anchored to adjacentstents (see FIGS. 6A and 6B), or in the alternative, have sufficientoverlap (in a non-fenestra section) to accommodate possible contractionsor expansions. Individual stent sections may have anchors that willprevent movement of that anchor point within the lumen walls, byanchoring to the lumen walls. For instance, in a three stent systemdeployed in the pelvic veins (see FIG. 1), maintaining the upper stentmodule's placement (module A) and lower stent module's placement (moduleC) are more critical. Anchoring both ends of the extended stentstructure firmly in the vein wall will prevent migration of thestructure two ends of the structure. As shown, in module A, the terminalend (which may not be interior to another stent, if the IVC module isnot needed) as shown, has a sealing ring or anchor (an expandablehelical ring) near the end of the stent containing the fenestrum,thereby maintaining the proper orientation and placement of thefenestrum. The middle module, module B, is not necessarily anchored,provided sufficient overlap is present to accommodate movement betweenadjacent stent modules. Lacking such overlap, an anchor or sealing ringis desired in the overlap region, with the sealing ring positioned onthe interior stent, either integrated into the stent sidewalls, or aseparate anchor ring positioned on the interior surface of the interiorstent. The sealing ring is designed to be more expansive than theremainder non-reinforced portions of the stent body; the sealing ringfirmly anchors the interior stent end to the exterior stent, and theexterior stent to the vein wall (see FIGS. 6A and 6B). The third stent(module C) has a reinforcing ring positioned at the lower end, anchoringthe lower terminus of the extended sting structure to the lumen walls.

As described, each stent module is radially expandable and has agenerally tubular body portion and first and second terminal endportions, with no liner or graft. In adjacent overlapped sections, whenanchoring is done, it is preferred that the interior stent module havemore spring force in a portion of the overlapped region to help ensurethat the stent modules are engaged with one another and resist relativemotion between the modules in the overlapped region.

The desired coupling or anchoring can be accomplished by having theanchoring stent end reinforced with slightly more expansive springforce, thereby mechanically assisting the coupling between the stentmodule and an adjacent stent or the lumen walls. This coupling can beaccomplished also by placement of a helical spring or sealing collar inthe interior of the interior stent module (which may be placed at thesame time or after placement of the stent). Hooks in the exteriorsurface of the interior stent walls have been used to couple stentsegments together—this arrangement is not preferred, as hooks can alsoengage vessel side walls, causing damage if the stent moves. When asealing collar, such as a helical coil, is used, the coil will be smallcompared to the stent module—for instance, the coil may be a section 2cm long, while the surrounding stent module may be 10 cm in length.Alternatively, the helical coil may be interwoven or integrated into thebody of the stent, for instance, by weaving a helical coil springthrough the terminal rings of the stent, or welding the helical coil tothe stent rings or terminal stent ring, or other such manufacturingtechnique (placement of a terminal expansive ring member on the stent isconsidered as sealing that end of the stent). Such additional expansiveforces on the terminal end of the stent body (or even on the interior ofthe stent body) will help lock the reinforced region with the contactingsurface. When only one end of the stent is “sealed” or reinforced, thestent ends will move (lengthen or contract) primarily only at theunsealed or non-reinforced end, when the stent body is subject tocompressive or tensile forces.

Generally, the beginning and landing sites have to be precise,therefore, the uppermost module should be deployed from the top (distal)of the catheter housing the stent, while the lowest module should bedeployed from the bottom (proximal) of the stent housing (like openingan umbrella, see FIG. 4B). Such deployment ensures accuracy in placementof the stent modules. It may be also be desirable to include helicalexpansive rings near a side fenestrum (such as near one side of theopening, or near both sides of the opening), to resist movement of thestent in the area of the fenestrum, as movement can cause misalignmentof the fenestrum with the opening in the vessel.

As described, modular sections of the stent structure, or stent modules,may be selectively combined to form a composite stent havingcharacteristics which are tailored to the specific requirements of thepatient. Each stent module can include reinforced sealing ends (one orboth) for coupling to adjacent modules with additional spring force.Alternatively, a particular stent module may have discrete “reinforced”sections (for instance, a section that is up to ½ the length of theoverall stent) either built into the stent module itself or provided bydeployment of an additional “reinforcing stent module” by catheter intothe interior of a previously deployed stent module. The supplemental“reinforcement stent,” a second stent generally smaller in length(without a liner or graft) that has the needed characteristics, may bedeployed internal to a stent module, or if reinforcement againstcompressive forces is desired, may be deployed first, and the stentmodule deployed through the reinforcement stent.

The stent modules can vary in axial length, cross-section, perimeter,resilient expansive force (spring force), flexibility, or other desiredcharacteristics, and also along the axis of each stent module. Selectionof appropriate stent modules and of the degree of overlap provides acustom fit stent. If a reinforced segment is desired, a separate shortstent, suitable to cover the area requiring reinforcement, may be added.The reinforce segment may have the same rigidity characteristics as theexterior stent, as in the region of the double stent, the effectivedevice wall thickness will be greater than that of the exterior stent,thus providing the desired additional resistance to compression. A stentmodule can be constructed to have a varying diameter along its length(e.g., a taper), to accommodate variation in the venous lumen diameter.This can be accomplished by varying the construction of the stent modulealong its length, such as by having the body of the stent constructed ofvarying diameter intermeshing or interlocking ring segments.

The modules are preferably individually introduced into a lumen systemof a patient's body one at a time so that the composite stent isassembled in situ. It is preferred that the ends of the stent modulescontain dense radio opaque material to assist in placement of themodules through fluoroscopy techniques. For instance, integrated helicalrings may be formed of radio opaque materials.

The invention claimed is:
 1. A stent system comprising a first andsecond expandable stent, each of said stents comprising a wall surfacewhen expanded having a hollow interior forming an interior surface andan exterior surface, each of said stents having a first terminal end anda second terminal end and a length defined by the distance between saidfirst and second terminal ends, each of said stents, along the length ofeach of said of stents, when expanded, are sized to be greater indiameter than the tubular lumen into which each of said stents isintended to be deployed, the first terminal end of said first stent (the“interior stent”) configured to be placed interior the second terminalend of the second stent (the “exterior stent”) and to resist sliding ofthe first terminal end of said interior stent with respect to saidsecond terminal end of said exterior stent when said interior stent isdeployed interior to said exterior stent, where the configuration toresist sliding comprises a reinforced section, said reinforced sectioncomprising an expandable helical coil not constrained by a liner, saidreinforced section providing expansive spring force greater than thatprovided by the second terminal end of said exterior stent.
 2. The stentsystem of claim 1 wherein said expansive helical coil is formed in orattached to the wall surface of said interior stent's first terminalend.
 3. The stent system of claim 1 wherein said expansive helical coilis separate from said wall surface of said interior stent.
 4. The stentsystem of claim 1 wherein one of said first or second stents has anopening in said wall surface sized, when expanded, to accommodate abranch vein opening in a venous segment.
 5. The stent system of claim 4,wherein said wall surface of said stent containing said opening furthercomprises a second reinforced section adjacent said opening.
 6. Thestent system of claim 1 wherein one of said first or second stents has atapered section.
 7. A stent system comprising a first and secondexpandable stent, each stent configured to expand in a vein comprising atubular tissue wall, each stent when expanded, having a wall surfaceforming an interior surface and an exterior surface, each of said stentshaving a first terminal end and a second terminal end and a lengthdefined by the distance between said first and second terminal ends,each of said first and second stents, when expanded and positioned at alocation in a vein, sized to have the respective exterior surface,substantially along the length of the respective stent, be in contactwith either the vein tissue wall or the other stent, whereby said firststent, at said first stent's first terminal end, overlaps said secondstent at said second stent's second terminal end, said first stent'sfirst terminal end being interior said second stent's second terminalend, said first stent's first terminal end further comprising areinforced section, said reinforced section not constrained by a linerand providing more expansive spring force than the second terminal endof said second stent.
 8. The stent system of claim 7 wherein saidreinforced section comprises an expandable helical coil or ring.
 9. Thestent system of claim 8 wherein said helical coil or ring is interwoveninto the first stent's first terminal end.
 10. The stent system of claim8 wherein said helical coil or ring is a separate ring or coil.